1. Field of the Invention
The present invention relates to a sheet post-processing apparatus for stapling sheets of paper after having formed images thereon and then folding the same in the middle to form a booklet and, more specifically, to a sheet post-processing apparatus in which the accuracy of middle folding is improved.
2. Description of the Related Art
Some of image forming apparatuses such as copying machines employ a sheet post-processing apparatus for performing a post-processing such as sorting or stapling sheets of papers formed with images thereon. Among others, a saddle unit which performs a saddle stitch process (staple) and then folding the sheets of paper in the middle to form a booklet is required the accuracy of the middle folding from the necessity of finish in good appearance.
The saddle unit is required specifically to allow easy and complete removal of jammed paper in case of a paper jam and to be able to fold the sheets of paper properly in the middle.
As a mechanism for releasing the paper jam, there is proposed a mechanism in which a stapler portion is configured as a single unit, and the unit is drawably mounted to a main body (for example, JP-A-2004-91129).
However, this technology has a problem that it is difficult to remove the sheet jammed in the unit and, when the sheet is forcedly pulled out, the accuracy in assembly of the unit is impaired.
As a mechanism for folding sheets of paper in the middle in good appearance, there is proposed a mechanism having a second folding roller which moves in the direction orthogonal to the direction of the middle holding, and the folding roller reciprocates the folded portion of the sheets of paper after having folded in the middle several times to ensure the folded state (for example, JP-A-2003-182928).
There is also proposed a technology to change the direction of movement of the second folding roller depending on the sheet size (for example, JP-A-2007-91469).
However, with these technologies, when the size of the sheets of paper is small, and when the bundle of sheets is thick, the force for ensuring the folded state is not transmitted sufficiently to the sheets of paper. Therefore, there arises a problem that the booklet after having folded in the middle is bulged.